A turbocompound system of an engine assists the engine by putting mechanical power into the crankshaft of the engine. The mechanical power is developed through an electric machine or device that acts as a motor and is connected to the crankshaft. The electrical power that drives the motor is produced by another electric machine or device associated with a turbocharger and that acts as generator. Typically, this generator operates as such by rotation of the turbocharger shaft. And the turbocharger shaft rotates in response to exhaust gases from the engine that turn a turbine. While the principal purpose of the turbocharger is to compress gases with a compressor for introduction into the engine cylinders (called “boost”), the turbocompound system provides an additional mechanism to recover energy that might otherwise be lost where the energy in the exhaust gases exceeds what is needed to drive the compressor.
A turbocompound system can also provide other advantages. The electric machine associated with the turbocharger may also act as a motor instead of a generator in certain instances. And the electric machine associated with the engine crankshaft can likewise operate as a generator. In instances where the turbine cannot provide sufficient mechanical power to drive the compressor to meet the needs of the engine, the crankshaft can drive its associated electric device as a generator. Power from the generator will drive the electric machine on the turbocharger shaft as a motor, thus providing additional energy to the drive the compressor and increase the compressed air flowing to the engine.
While the adaptability of such a turbocompound system is apparent, the control of the system itself is critical to its capability to recover energy from exhaust gases that would otherwise be lost, to improve engine response under various conditions, and/or fulfill other purposes such as driving additional electrical devices. At the same time, these opportunities must be carefully managed, so that overall system efficiency is achieved.
An example of one turbocompounding system is in U.S. Pat. No. 5,678,407 issued to Hara on Oct. 21, 1997. The system disclosed uses calculated and actual engine values to determine whether the engine and the turbocharger mounted generator/motor are under certain conditions. Depending upon the condition, the generator/motor may be shifted from the generator mode to the motor mode or vice versa. The control system is designed to prevent abrupt mode changes, avoiding consequent abrupt load changes on the engine for smooth operation.
While the disclosure of the '407 patent affects the control of the engine, the aspect of control is directed to the acceleration mode of the engine. Other considerations and engine parameters are important to improve overall system efficiency, providing a control system that can maximize gains in efficiency. The disclosed invention is directed to overcoming one or more of the limitations discussed above.